\(\delta \)  Sct stars pulsating in the radial fundamental (F) and/or in the first overtone (1O) mode show a
period–luminosity relation, albeit with a larger scatter than for Cepheids (see e.g. [1]},
[2]}, [3]}, [4]}, [5]}). For these modes, the pulsation
constant \(Q \equiv P\sqrt{\bar{\varrho }/\bar{\varrho }_{\odot }}\)  is more or less constant over the instability strip,
and the dominant stellar property that correlates with the stellar mean density is the luminosity \(L\) ,
hence the PL relation [6]}.
